1. Field of the Invention
The present invention relates to a fluoride refining method and a fluoride crystal manufacturing method which are suitable for various optical elements, lenses, window materials, prisms or the like used for a light of a predetermined wavelength selected from a wide wavelength range between the vacuum ultraviolet region and the far infrared region. Further, the present invention relates to a technical field of optical parts and an aligner using the same.
2. Related Background Art
A fluoride crystal such as one of calcium fluoride has a high transmittance in a wide wavelength range between the vacuum ultraviolet region and the far infrared region, and is widely used for various optical elements, lenses, window materials, prisms or the like. Among them, fluorite (calcium fluoride) which is excellent in transmission characteristics at shorter wavelength is useful as an optical part for excimer laser. In particular, a calcium fluoride crystal of 70% or more in its internal transmittance to light of 135 nm in wavelength has a superior durability to an ArF excimer laser, and its transmission characteristics are less degraded by repeated irradiation with high-output laser.
For such a fluoride crystal, a process for melting and refining a raw material is required to increase bulk density of the raw material and remove impurities of the raw material. In this refining process, a scavenger which is a metal fluoride must be added to a raw material to remove an oxide produced by a reaction of the raw material with water or the like, or impurities in the raw material. For example, when the fluoride crystal is calcium fluoride, and when the scavenger is solid ZnF2, CaO produced by a reaction of the raw material with water reacts with ZnF2 to be converted to CaF2, while the scavenger becomes ZnO to evaporate during melting of the raw material.
When a high-quality fluoride crystal with superior optical performance is to be obtained, the above mentioned refining process must be repeated several times. In addition, a raw material of a high purity with less impurities must be employed.
When a crystal is manufactured using the thus obtained fluoride crystal block as a raw material, a fluoride crystal with very superior optical performance such as transmission characteristics is obtained.
On the other hand, there is a method in which a gaseous scavenger is used instead of a solid scavenger. A reactive gas that can be used as a gaseous scavenger includes hydrogen fluoride gas or carbon fluoride gas.
However, since the solid scavenger contains a metal element, there is a case where the metal element remains in the crystal, which affects the transmission characteristics. Therefore, when the addition amount of the solid scavenger is decreased, or when a cover of a crucible is bored to increase degassing properties, the metal element from the scavenger hardly remains in the crystal. However, if the addition amount is too small, the effect of the scavenger decreases. In addition, if the degassing properties are increased excessively by boring the crucible cover excessively, the scavenger is discharged to the outside of the crucible before exhibiting its effect. Thus, improvement in the transmission characteristics is prevented by contamination of the fluoride with an oxide or the like. Therefore, it becomes important to define an optimum addition amount of a scavenger or to design a crucible in such a shape as to optimize the degassing properties. However, these vary depending on the content of water or impurities of a raw material, and lacks general-purpose use.
On the other hand, a hydrogen fluoride gas of the gaseous scavenger has corrosion properties, and sulfur tetrafluoride and boron trifluoride have toxicity. Thus, the management cost is increased. In addition, the gas is dissolved in the melt and is often incorporated as air bubbles into the crystal (Guggenheim, J. Appl. Phys. 34, pp. 2482-2485 (1963)), and satisfactory transmission characteristics are hardly obtained.
Further, when a carbon fluoride gas such as methane tetrafluoride (CF4) is used as a reactive gas, a crystal with superior transmission characteristics can be obtained. However, these gases are very stable in the air. Further, these gases are greenhouse effect gasses causing global warming as with carbon dioxide and must be removed by decomposition with plasma or the like, thereby increasing the management cost.
An object of the present invention is to provide a fluoride refining method and a fluoride crystal manufacturing method that have great general-purpose properties and can reduce the manufacturing cost.
Another object of the present invention is to provide at a low cost a fluoride crystal, an optical part and an aligner the transmission characteristics of which are hardly degraded even when repeatedly irradiated with a high-output light of a short wavelength for a long term.
The method of refining a fluoride according to the present invention comprises the heating step of heating a solid scavenger-added fluoride raw material to melt the raw material and the cooling step of cooling the molten fluoride material to solidify the melt, wherein the environment of a chamber housing the fluoride raw material is changed to such an environment that a gas in the chamber is discharged to the outside of the chamber more easily than the environment before the change, during the heating step.